The basic electrostatic mechanism of electromechanical transduction has been known and applied to various uses for over two hundred years. It was not until the period following World War II, however, that the availability of synthetic materials such as polyester film, polyvinyl-chloride insulation and other synthetic plastics having suitable properties made practical electrostatic loudspeakers possible. Recent embodiments of such electrostatic loudspeakers employ a polyester film diaphragm less than 17 microns in thickness with an extremely thin, applied electrically conductive coating, the diaphragm being suspended between two accoustically transparent plates, usually insulated with polyvinyl-chloride coating. These stator plates are ordinarily spaced so as to leave a diaphragm excursion gap of a few millimeters. A polarizing voltage of a few thousand volts D.C. is applied to the conductive coating on the diaphragm to spread charges uniformly over its surface. High voltage audio signals are applied to the outer opposed stator plates, usually in push-pull fashion for most linear operation. The advantages of such electrostatic transducers are uniquely desirable for the following reasons:
(1) If a diaphragm charge is kept constant, which is easy to do, the forces appearing on the diaphragm vary only with the audiovarying electric fields on the stators, and do not depend on diaphragm position in the intervening space between the stators. PA1 (2) Since the charges on the diaphragm reacting to the electrostatic field are typically less than a wavelength of light apart, the induced forces will be substantially uniform over the entire diaphragm surface. PA1 (3) The force per unit area (pressure) created on the diaphragm will be the same for any size of transducer, all other parameters being held equal.
These ideal properties are shared by no other known audio transducer, and can result in highly accurate sound reproduction spanning the entire audio spectrum from 20 Hz to 20 kHz utilizing one or more electrostatic elements, each of which operates throughout the entire range of audio frequencies.
A practical full-range-element electrostatic loudspeaker will typically require a total diaphragm surface area of 0.5 to 1.0 square meters for good acoustic impedance match if high efficiency and output are to be obtained. This area is usually subdivided into several bays to solve problems of diaphragm resonant frequency, stability and dispersion. At the same time, low mass per unit area of the diaphragm is required for accurate high frequency reproduction. Such practical electrostatic loudspeakers will typically present a stator-to-stator capacitance of about one nanofarad (10.sup.-9 Farad) per square meter.
Despite their commanding natural advantages, electrostatic loudspeakers to the present time represent an almost negligible fraction of existing loudspeakers in use. The reasons for such general lack of acceptance of electrostatic loudspeakers as a practicable competitor with electrodynamic loudspeaker systems, for example, resides mainly in the difficulties in designing a satisfactory audio power drive interface between existing audio power amplifiers having ordinary low signal voltage output characteristics and the electrostatic transducer. The first problem with such an electrostatic transducer driving interface resides in the difficulty in achieving accurate high-voltage audio drive signals. The second difficulty in interface design resides in the capacitive nature of the electrostatic transducer's load characteristic, reflecting radical impedance changes over the approximately 1,000:1 range of the audio frequency band. The third difficulty resides in the requirement for significant spectral equalization for the electrostatic transducer's voltage-to-acoustic transfer characteristic spanning a ratio of more than ten decibels. All of these design criteria must be incorporated in the interface driving means if a practical full-range-element electrostatic loudspeaker system is to be achieved, and must be effective at modest cost to be competitive with electrodynamic loudspeaker systems, for example, which presently dominate the field.
Various attempts to design a power amplifier interface for full-range-element electrostatic loudspeakers and to be driven by ordinary low voltage output power amplifiers which are commonly available at modest cost, and at the same time satisfactorily meet the above described design criteria, have been unsuccessful. Principally, such attempts have involved the use of a single audio step-up transformer to raise the low voltage output signal of an ordinary power amplifier by a factor of about 100:1 for proper voltage drive of the electrostatic loudspeaker. Studies of transformer physics and scaling laws, however, demonstrate that it is impractical to make one transformer accomplish this magnitude of step-up, working into a one nanofarad load over the full range of the audio band. Such systems are characterized by inefficiency, poor spectral balance, and large, very costly transformers. FIG. 1 illustrates this classical approach in the prior art.
The use of two or more transformers to extend flat-amplitude band-pass in a general purpose transformer coupling system is also known, as described, for example, in U.S. Pat. No. 3,231,837 to O'Meara. The resulting, flat, all-pass characteristics detailed in such multiple transformer systems of the type described in the O'Meara patent, however, are not suited to the resolution of the above described matching and full range driving problems peculiar to electrostatic loudspeakers. In particular, no provision is made for correction for the serious impedance fluctuation character of the electrostatic transducer with frequency; no provision is made to fulfill the important need for spectral equalization in which drive voltages are required to vary over more than a ten decibel range in the audio spectrum; and no provision is made for achieving acceptable drive efficiency at high frequencies.
Because of the above described unresolved problems heretofore experienced in the design of interface circuitry driven by existing low voltage audio amplifiers, full-range-element electrostatic loudspeakers have been most successfully driven by specially designed and dedicated high-voltge amplifiers supplying audio signals of about two orders of magnitude higher amplitude than commonly available in existing amplifiers. Such dedicated high voltage amplifiers invariably incorporate equalized pass response networks. Because of their comparative high cost and specialized nature, they have enjoyed only minimal acceptance by the general public for use in high fidelity audio systems utilizing electrostatic speakers.